Thaddeus Ku Han-seon was born in Minarigol, Haman, Gyeongsang-do (now, Pyeongrim-ri, Daesan-myeon, Haman-gun, Gyeongnam) to a middle-class family. Thaddeus Ku was an intelligent young boy and read many books. He sometimes became fooled by magical practices before he became a Catholic.
One day he met a Catholic by chance and heard about the Catholic religion. He accepted it without delay and was baptized by Bishop St. A. Daveluy. He devoted himself to practicing his religion for ten years, and was selected to be an altar server for Father F. Ridel. Sometimes, he accompanied Father Ridel to Geoje Island for missionary work.
Thaddeus Ku was arrested by the police of Jinju when the Byeongin Persecution broke out in 1866, on his way home after saying good-bye to Father F. Ridel. He was taken to the government office and there he had to go through interrogation and severe torture. But he never renounced his faith in God. While in prison, he wrote the substance of the Catholic doctrine and sent it to the wife of the magistrate of Jinju.
When the magistrate‘s wife read Thaddeus Ku`s writing, she asked her husband to release him. But the magistrate, who became very angry at his wife`s request, ordered that the criminal be pulled out of prison and beaten severely. Despite this Thaddeus Ku did not even utter a sound or a groan. Then the magistrate became even more angry and scolded the prison guards, saying, `You are not beating him hard enough. Beat him with all your strength.` The guards said, `We have beaten him with great strength. If we beat him more, he will die.`
The magistrate asked Thaddeus Ku; "Why don`t you groan or make a sound?" Thaddeus Ku answered; "My old mother is standing outside the gate. If I give a groan, she will hear me and will faint. This is the reason why I do not groan." The magistrate asked again; "Then why do you believe in the Catholic religion?" Thaddeus Ku answered; "The Catholic Church teaches us to practice filial piety toward our parents. That`s why I believe in God."
After cruel punishment, Thaddeus Ku was released and returned home. But he died seven days later, due to the severe beatings and torture. Thaddeus Ku was 22 years old.
It is said that when he died a martyr the Chinese character (品) was printed on his forehead in red. His family came to take his body and buried it near his hometown.